Geoscience Reference
In-Depth Information
means you should be thinking “ornithopod” or “theropod” for nearly all three-toed
fossil tracks in rocks from dinosaur times. But also keep in mind how some orni-
thopods also left small front-foot impressions, a result of
sometimes
walking quad-
rupedally. Theropods, in contrast, almost always moved on just two legs, although,
as you learned earlier, a few rare instances of hand impressions show up in trace
fossils made where they stopped briefly to sit.
So now let's say you found some three-toed dinosaur tracks and you want to
figure out whether these are from a theropod or an ornithopod. The most basic way
to tell the difference is to measure the track length and width, then compare the
two.Onaverage,theropodtracksarelongerthantheyarewide,whereasornithopod
tracks are wider than they are long. This means a length:width ratio for a theropod
track will be >1.0, and <1.0 for an ornithopod. The tracks you've examined have
ratios of 0.8 to 0.9, so these
must
be from ornithopods! Right?
Oh, if only ichnology were so simple, where we could all be so satisfied with
ourresults,teemingandpreeningwithconfidence.Okay,timeforanexerciseinhu-
mility. Let's go through a few questions and see how you do in answering them:
• How would you describe the toes relative to the overall length of the track:
thin, medium, or fat?
• Did those toes end with sharp clawmarks or blunt ones?
• What did the “heel” (posterior of the track) look like?
• Did you take into account how the substrate preserving a track, which
might have been wet mud, dry sand, or moist muddy sand, might have af-
fected the overall outline of a track?
• Did you think about how the dinosaur stopping suddenly, turning to the
right, or moving its head might have distorted the track outline?
• Did the dinosaur, when extracting its foot from the mud or sand, cause sed-
iment to collapse into the toe impressions and thus change the character of
the track?
• Did you also think of how the substrate drying out might have changed a
track outline before it was fossilized?
